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# Gravity

yagnyavalkya
Instantaneous action of gravity
According to Albert Einstein's theory of special relativity, instantaneous action at a distance was seen to violate the relativistic upper limit on speed of propagation of information. For example if a massive object was removed will the effect be instantaneous or not
If so then meaning information had been transmitted faster than the speed of light. This violates the relativistic upper limit on speed of propagation of information
Speed of expansion of space is known to be faster that light
Is it possible that changes in the curvature of space also can be faster that light
Bikerman
Most physicists I know are pretty sure that gravity propagates at c or thereabouts.

The Chinese have produced some evidence for a value between 0.93c and 1.05c which is right in the sweet-spot one would predict if it DOES travel at c.

http://www.spacedaily.com/reports/Chinese_scientists_find_evidence_for_speed_of_gravity_999.html
kelseymh
 yagnyavalkya wrote: Instantaneous action of gravity According to Albert Einstein's theory of special relativity, instantaneous action at a distance was seen to violate the relativistic upper limit on speed of propagation of information. For example if a massive object was removed will the effect be instantaneous or not If so then meaning information had been transmitted faster than the speed of light. This violates the relativistic upper limit on speed of propagation of information

Correct. Which is why moving a massive object generates gravitational waves.

 Quote: Speed of expansion of space is known to be faster that light

Um. No, it's not. The rate of cosmological expansion is, in the non-relativistic limit, about 80 km/s per megaparsec of separation. 80 km/s is quite slow on astronomical scales (the orbital speed of the sun around the galaxy is ~200 km/s, for example).

Your statement is a common misconception, and results from applying a low-velocity linear approximation outside its domain of validity.

 Quote: Is it possible that changes in the curvature of space also can be faster that light

Anything is "possible," but if you are trying to come up with some reason why general relativity is wrong, you're going to have to do better than that. Making elementary mistakes means it's much more likely that you are wrong than that extremely well-tested and well-understood physics is wrong.
yagnyavalkya
 kelseymh wrote: Anything is "possible," but if you are trying to come up with some reason why general relativity is wrong, you're going to have to do better than that. Making elementary mistakes means it's much more likely that you are wrong than that extremely well-tested and well-understood physics is wrong.

Thanks
I am not trying to come up with a reason "why general relativity is wrong' just asking a hypothetically question GT has been tested rigorously and found to be correct
Recessional velocities is faster that light for some very distant galaxies which are more than about 14 billion light-years from us
What will be the speed of curvature change there
davorin
According to current theory, the speed of light is the maximum speed at which you can travel.
But, there is some theory, that says that the universe is composed of multiple dimensions and is curved.
So, if you con't travel faster than speed of light, you can go on subspace, and than travel more than speed od light.
And that something is measured to have traveled faster than the speed of light, could be a faulty of the Measuring Instruments or delay in the wires.
ocalhoun
kelseymh wrote:

 Quote: Speed of expansion of space is known to be faster that light

Um. No, it's not. The rate of cosmological expansion is, in the non-relativistic limit, about 80 km/s per megaparsec of separation. 80 km/s is quite slow on astronomical scales (the orbital speed of the sun around the galaxy is ~200 km/s, for example).

Your statement is a common misconception, and results from applying a low-velocity linear approximation outside its domain of validity.

So, wait, there is no (Hubble distance was it called?)?
But I thought things receding progressively faster as they are progressively further away was pretty much taken as fact, and that at sufficient distance, that speed becomes (apparently) greater than the speed of light.
(Though nothing is actually moving that fast, just empty space in between increasing at that rate.)
Does the expansion rate become non-linear over extremely large distances then?
*tries to avoid figuring out how that would work until getting an answer to this... I think I might break my mind.*

 davorin wrote: So, if you con't travel faster than speed of light, you can go on subspace, and than travel more than speed od light.

You may get from point A to point B faster than light would, but you still don't travel faster than light.
You're traveling slower than light over a shorter distance. ... still moving slower, but taking a shortcut to get there first.
kelseymh
ocalhoun wrote:
kelseymh wrote:

 Quote: Speed of expansion of space is known to be faster that light

Um. No, it's not. The rate of cosmological expansion is, in the non-relativistic limit, about 80 km/s per megaparsec of separation. 80 km/s is quite slow on astronomical scales (the orbital speed of the sun around the galaxy is ~200 km/s, for example).

Your statement is a common misconception, and results from applying a low-velocity linear approximation outside its domain of validity.

So, wait, there is no (Hubble distance was it called?)?

The Hubble distance is the low-velocity approximation: you take Hubble's constant and pretend it works out to infinity.

 Quote: But I thought things receding progressively faster as they are progressively further away was pretty much taken as fact, and that at sufficient distance, that speed becomes (apparently) greater than the speed of light.

Yes and no. Things recede faster the farther away they are (which is how we use redshift to infer distance). If you us the redshift/distance relationship without accounting for relativity, then you'll "discover" that out around redshift z=1, everything is superluminal. But that's because the z = v/c relationship only holds at low velocities.

 Quote: Does the expansion rate become non-linear over extremely large distances then?

Yes and no. The expansion rate now is the same everywhere (a corollary of the Copernican principle). However, when we look out into the universe, at large distances the expansion rate we would observe between distant objects is different than H_0, because we're looking back in time.
ocalhoun
 kelseymh wrote: because we're looking back in time.

Ah, right. That's the factor I was missing.
kelseymh
ocalhoun wrote:
 kelseymh wrote: because we're looking back in time.

Ah, right. That's the factor I was missing.

Technically, the quantity you really care about is the scale factor, written as a(t), where 't' is cosmological time, the hypothetical clock in the rest frame of the Universe. This is the quantity you can use with the metric to see how spacetime is changing. Specifically, the proper distance between objects at rest is d(t) = a(t) d_0, where d_0 is their separation at some reference time "0" (see below).

The Hubble factor is (da/dt) / a(t); the "Hubble constant" we measure, H_0 is just (da/dt)|t=0 / a(0), where "0" means "now."